The present invention relates to turbine bucket design and, more particularly, to a core design that allows for independent wall thickness control at the airfoil leading edge and trailing edge of a cooled bucket.
The efficiency of a gas turbine is related to the operating temperature of the turbine and may be increased by increasing the operating temperature. As a practical matter, however, the maximum turbine operating temperature is limited by high temperature capabilities of various turbine elements. Since engine efficiency is limited by temperature considerations, turbine designers have expended considerable effort toward increasing the high temperature capabilities of turbine elements, particularly the airfoil shaped vanes and buckets upon which high temperature combustion products impinge. Various cooling arrangements, systems and methods extend operating temperature limits by keeping airfoils at lower temperatures. The cooling of airfoils is generally accomplished by providing internal flow passages within the airfoils. These serpentine cooling passages accommodate a flow of cooling fluid.
All portions of the turbine airfoils should be adequately cooled. In particular, adequate cooling should be provided for leading and trailing edges of the airfoils, because these portions are normally the most adversely affected by high temperature combustion gases. Known cooling configurations tend to inadequately cool the airfoils, especially at leading and trailing edges of the airfoils.
It would be helpful for cooling if the wall thicknesses of the buckets at the leading and trailing edges were optimized. Typically, a one-piece core is supported in a casting die, and prior to the casting procedure, the core is positioned so that the end product wall thicknesses at the leading and trailing edges of the bucket are appropriate to accommodate design considerations. In this context, however, through positioning of the core in the casting die, the optimal positioning of one of the leading edge or the trailing edge for appropriate wall thickness results in sacrificing optimal positioning of the other of the leading or the trailing edge, and the end product may not meet desired part life requirements due to inadequate cooling capabilities.
In an exemplary embodiment of the invention, a core for use in casting a turbine bucket including serpentine cooling passages includes a leading edge core section positionable in a casting die, and a trailing edge core section separate from the leading edge core section and separately positionable in the casting die. Each of the leading edge core section and the trailing edge core section preferably includes serpentine cooling passages.
In another exemplary embodiment of the invention, a two-piece core for use in casting a turbine bucket including serpentine cooling passages is provided, wherein each of the pieces is separately positionable in a casting die for independently controlling wall thicknesses at a leading edge and a trailing edge of the turbine bucket.
In another exemplary embodiment of the invention, a method of casting a turbine bucket includes controlling wall thicknesses at a leading edge and a trailing edge of the turbine bucket independent of each other. In this context, the controlling step preferably includes positioning a leading edge core section in a casting die and separately positioning a trailing edge core section in the casting die.